Tunable external cavity laser

ABSTRACT

External cavity lasers apparatus and methods that allow fast tuning, high wavelength stability, low cavity losses, and form factors that are comparable to solid state, fixed wavelength lasers. The apparatus comprise a gain medium emitting a light beam, a tunable wavelength selection element positioned in the light beam and configured feed back light of a selected wavelength to the gain medium, and a microelectromechanical systems (MEMS) actuator element operatively coupled to the tunable wavelength selection element. The MEMS actuator element may be configured to actuate the tunable wavelength selection element according to a first degree of freedom to select the wavelength of the feedback to the gain medium, and to actuate the actuate the tunable wavelength selection element according to a second degree of freedom to provide phase control of the feedback. The MEMS actuator element and tunable wavelength selection element may additionally be configured such that actuation of the tunable wavelength selection element with respect to a third degree of freedom provides a selectable level of attenuation of the feedback to the gain medium.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to U.S. patent application Ser. No.09/900,373, filed on Jul. 6, 2001, and incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] The demand for increased bandwidth in fiberoptictelecommunications has driven the development of sophisticatedtransmitter lasers usable for dense wavelength division multiplexing(DWDM) systems wherein multiple separate data streams propagateconcurrently in a single optical fiber. Each data stream is created bythe modulated output of a semiconductor laser at a specific channelfrequency or wavelength, and the multiple modulated outputs are combinedonto the single fiber. The International Telecommunications Union (ITU)presently requires channel separations of approximately 0.4 nanometers,or about 50 GHz, which allows up to 128 channels to be carried by asingle fiber within the bandwidth range of currently available fibersand fiber amplifiers. Greater bandwidth requirements will likely resultin smaller channel separation in the future.

[0003] Telecom DWDM systems have largely been based on distributedfeedback (DFB) lasers. DFB lasers are stabilized by a wavelengthselective grating that is predetermined at an early step of manufacture.Unfortunately, statistical variation associated with the manufacture ofindividual DFB lasers results in a distribution of (wavelength) channelcenters. Hence, to meet the demands for operation on the fixed grid oftelecom wavelengths (the ITU grid), DFBs have been augmented by externalreference etalons and require feedback control loops. Variations in DFBoperating temperature permit a range of operating wavelengths enablingservo control; however, conflicting demands for high optical power, longlifetime, and low electrical power dissipation have prevented use inapplications that require more than a single channel or a small numberof adjacent channels.

[0004] Continuously tunable external cavity lasers have been developedto overcome the limitations of individual DFB devices. Tunable externalcavity lasers, in order to provide effective side mode suppression andwavelength stability, require very stringent manufacturing tolerances.In order to meet these tolerances, expensive custom-made components aretypically required for the external cavity lasers. Tuning has relied onthe use of stepper motors to mechanically components, which reduces formfactor, introduces vibration and shock sensitivity, reduces usefullifetime due to motor component wear, and increases the overall size andcomplexity of the external cavity lasers.

[0005] There is accordingly a need for an external cavity laser that iscompact in size and has a small form factor, that is of simple,inexpensive construction, that provides for effective side modesuppression and wavelength stability during operation, that has reducedcavity loss and increased output power, and which has loose machiningtolerances. The present invention satisfies these needs, as well asothers, and overcomes the deficiencies found in the background art.

SUMMARY

[0006] The invention provides external cavity lasers apparatus andmethods that allow fast tuning, high wavelength stability, low cavitylosses, and form factors that are comparable to solid state, fixedwavelength lasers. The apparatus of the invention comprises a gainmedium emitting a light beam, a tunable element positioned in the lightbeam and configured feed back light of a selected wavelength to the gainmedium, and a microelectromechanical systems (MEMS) actuator elementoperatively coupled to the tunable element. The MEMS actuator elementmay be configured to actuate the tunable element according to a firstdegree of freedom of movement to select the wavelength of the feedbackto the gain medium, and to actuate the tunable element according to asecond degree of freedom of movement to provide phase control of thefeedback. The MEMS actuator element and tunable element may additionallybe configured such that actuation of the tunable element with respect toa third degree of freedom of movement provides a selectable level ofattenuation of the feedback to the gain medium.

[0007] The tunable element and MEMS actuator are configured to provideorthogonalized wavelength selection control and phase control of thefeedback from the tunable element to the gain medium according toindependent orthogonalized positional adjustments to the tunableelement. In other words, wavelength tuning is uncoupled or decoupledfrom tuning of the external cavity tuning, such that the tuningmechanisms for wavelength selection and external cavity lengthadjustment operate independently or orthogonally with respect to eachother. The adjustment of the wavelength passband thus has minimal effecton the effective cavity length, and adjusting the effective cavitylength has minimal effect on the passband of the tunable element.

[0008] In certain embodiments, the tunable element may comprise amovable grating that is positioned in the light beam and configured toselectively feed back light to the gain medium according to positioningof the grating by the MEMS actuator. The grating is rotatable about afirst axis to provide wavelength selection of the light fed back to thegain medium, and is translatable along a second axis to provide phasecontrol of the light fed back to the gain medium. The rotationaladjustment about the first axis to control wavelength selection isorthogonalized with respect to the translational adjustment along thesecond axis to control external cavity length, such that adjusting thegrating for wavelength selection does not effect, or minimally effects,phase adjustment. Similarly, translatational adjustment of the gratingalong the second axis to provide phase control does not effect, orminimally effects, wavelength selection. The grating may additionally berotatable about a third axis to provide attenuation control to the lightfed back to the gain medium. The first axis may be parallel orsubstantially parallel to the grating face of the movable grating, andthe second axis may be perpendicular or substantially perpendicular tothe grating face and first axis. The third axis may be substantiallyparallel to the grating face, and substantially perpendicular to thefirst and second axes.

[0009] In some embodiments, the movable grating is a reflective gratingand, together with a reflective facet of the gain medium, defines anexternal laser cavity. The grating may be etched or otherwise formedonto a MEMS mirror surface. The grating may be positioned with respectto the reflective facet of the gain medium such that external lasercavity is dimensioned to suppress lasing modes at wavelengths other thana selected wavelength. Specifically, the grating and gain medium may bepositioned such that the external laser cavity is of sufficiently shortlength that the external cavity axial modes are spaced sufficiently farapart such that unwanted mode hopping from a selected wavelength to anexternal cavity mode will not occur during laser operation. Theapparatus may, in certain embodiments, also comprise a mode filteringelement positioned in the optical path, which may be in the form of anetalon configured to define a plurality of transmission peakscorresponding to selectable feedback wavelengths.

[0010] In other embodiments, the tunable element may comprise a movablemirror together with a stationary grating, with the movable mirroroperatively coupled to the MEMS actuator element. The mirror isrotatable about a first rotational axis to control feedback wavelengthand translatable along a second axis to control feedback phase. Incertain embodiments, the mirror may additionally be rotatable about athird axis to control level of feedback attenuation.

[0011] The methods of the invention comprise emitting a light beam by again medium, positioning a tunable element in the light beam, couplingthe tunable element to a microelectromechanical (MEMS) actuator, feedingback light to the gain medium by the tunable element, and positionallyadjusting the tunable element with respect to a first degree of freedomof movement, via the MEMS actuator, to select wavelength of the lightfed back to the gain medium. The methods may additionally comprisepositionally adjusting the tunable element with respect to a seconddegree of freedom of motion to adjust phase of the light fed back to thegain medium. The positional adjusting with respect to the first andsecond degrees of freedom may be carried out orthogonally, such thatpositional adjustment of the tunable element to adjust wavelength doesnot affect phase adjustment provided by the tunable element. The methodsmay further comprise positionally adjusting the tunable element withrespect to a third degree of freedom of movement to control attenuationof the light fed back to the gain medium.

[0012] The positioning of the tunable element in the light beam may incertain embodiments comprise positioning a reflective grating in thelight beam. The grating may be etched, engraved or embossed or otherwiseformed, using photolithographic or other technique, onto the surface ofa MEMS-movable mirror that is coupled to a MEMS actuator. Positionallyadjusting the grating to select wavelength of the feedback light maycomprise rotatably actuating the grating with respect to a first axisthat is parallel or substantially parallel to the grating face.Positionally adjusting the grating to select or adjust the phase of thefeedback light may comprise translating the grating with respect to asecond axis that is perpendicular or substantially perpendicular to agrating face thereof. The first and second axes are configured such thatwavelength selection and phase selection are orthogonalized.Positionally adjusting the grating to control attenuation of orotherwise control the optical power of the feedback light mayadditionally comprise rotatably actuating the grating with respect to athird axis that is substantially perpendicular to the first and secondaxes.

[0013] In certain embodiments, the positioning of the grating in thelight beam may comprise positioning the grating such that the gratingand an reflective facet of the gain medium define an external lasercavity that is dimensioned to suppress lasing modes at wavelengths otherthan a selected wavelength. The grating and gain medium may bepositioned such that the external laser cavity is of sufficiently shortlength that the external cavity axial modes are spaced sufficiently farapart such that unwanted mode hopping from a selected wavelength to anexternal cavity mode will not occur during laser operation. In someembodiments, the method may comprise positioning a mode filteringelement in the light beam, and suppressing feedback at unselectedwavelengths with the mode filtering element.

[0014] The apparatus and methods of the invention provide externalcavity lasers that can be manufactured and assembled with relaxedtolerances and inexpensive components than is presently possible. Theuse of a MEMS actuator for positioning of a tunable element as providedby the invention allows shorter external cavity dimensions and smallerpackage sizes than have previously been achieved. In certainembodiments, the external cavity may be of sufficiently small dimensionthat effective suppression of unwanted wavelengths is achieved withoutthe use of an intracavity filter or mode suppression element. Themultiple degrees of freedom of movement of the tunable element allowwavelength selection, phase control and output power control duringlaser operation by appropriate actuation of the tunable wavelengthreflection element. Adjustment to provide wavelength selection and phasecontrol may be carried out independently or orthogonally, such thatadjustments to a tunable element to provide wavelength selectionminimally affect external cavity length or phase control adjustment. Theshort external cavity length and use of MEMS actuation also allowsdynamic provisioning and rapid tuning or adjustment of outputwavelength, feedback phase, and output power during laser operation.These and other objects and advantages of the invention will be apparentfrom the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The invention will be more fully understood by reference to thefollowing drawings, which are for illustrative purposes only.

[0016]FIG. 1 is a schematic illustration of a tunable external cavitylaser apparatus in accordance with the invention.

[0017]FIG. 2 is a top plan view of the tunable external cavity laserapparatus of FIG. 1 shown mounted on a sled in a hermetically sealedcontainer.

[0018]FIG. 3 is a perspective view of the tunable external cavity laserapparatus of FIG. 2.

[0019]FIG. 4 is a top plan view of the grating and MEMS actuator of thetunable external cavity laser apparatus of FIG. 2.

[0020]FIG. 5 is a schematic illustration of another embodiment of atunable external cavity laser apparatus in accordance with theinvention.

[0021]FIG. 6 is a schematic illustration of another embodiment of atunable external cavity laser apparatus in accordance with theinvention.

[0022]FIG. 7 is a top plan view of the tunable external cavity laserapparatus of FIG. 6 shown mounted on a sled in a hermetically sealedcontainer.

[0023]FIG. 8 is a top plan view of the tunable external cavity laserapparatus of FIG. 7 shown with the grating oriented such that thegrating face is substantially perpendicular to the face of the movablemirror.

[0024]FIG. 9 is a top plan view of an external cavity laser apparatus inaccordance with the invention with a shortened external cavity lengthand without an intracavity mode suppression filter.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Referring more specifically to the drawings, for illustrativepurposes the present invention is embodied in the apparatus shown inFIG. 1 through FIG. 9. It will be appreciated that the apparatus mayvary as to configuration and as to details of the parts, and that themethod may vary as to details and the order of the acts, withoutdeparting from the basic concepts as disclosed herein. The invention isdisclosed primarily in terms of use with an external cavity laser. Theinvention, however, may be used with various types of laser devices andoptical systems. It should also be understood that the terminology usedherein is for the purpose of describing particular embodiments only, andis not intended to be limiting, since the scope of the present inventionwill be limited only by the appended claims. The relative sizes ofcomponents and distances therebetween as shown in the drawings are inmany instances exaggerated for reason of clarity, and should not beconsidered limiting.

[0026] Referring now to FIG. 1, there is shown an external cavity laserapparatus 10 in accordance with the invention. The apparatus 10 includesa gain medium 12 and a tunable wavelength selection element that isshown in FIG. 1 as a reflective grating 14. Gain medium 12 may comprisea conventional Fabry-Perot diode emitter chip and has an anti-reflection(AR) coated front facet 16 and a reflective or partially reflective rearfacet 18. Grating 14 is reflective, and an external laser cavity 22 isdefined by rear facet 18 and the surface or face 20 of grating 14. Gainmedium 12 emits a coherent light beam 24 from front facet 16 that iscollimated by lens 26 to define an optical path 28. Grating 14 in otherembodiments may comprise a transmissive grating, a prism, aninterference filter, or other tunable element capable of providingwavelength selection.

[0027] Gain medium 12 emits an output beam 30 from facet 18, which iscollimated by lens 32 along an output path 34. The beam 30 from path 34is focused by lens 36 into an optical fiber 38. An optical isolator 40is positioned in optical path 34 between lenses 32, 26 to prevent returnof light from fiber 38 into gain medium 12. A coarse spectrometer (notshown) may also be positioned in output path 34 between lenses 32, 26 toprovide monitoring of output wavelength during the operation of theapparatus 10.

[0028] Grating 14 is operatively coupled to a microelectromechanicalsystem or MEMS actuator element 29 that provides for positionaladjustment of grating 14 during the operation of laser apparatus 10 asdescribed further below. The terms “microelectromechanical systemactuator” and “MEMS actuator” as used herein refer to actuator devicesbased on microsystems technology that are fabricated usingmicromachining techniques such as those used in the semiconductorindustry. Numerous MEMS actuator devices that can provide selectiveactuation of components are known and commercially available. The use ofMEMS actuation of a tunable wavelength selection element in an externalcavity laser in accordance with the invention offers several advantages,also discussed more fully below, that have heretofore not been availablein external cavity lasers. In certain embodiments, other actuatorsystems, such as stepper motors, voice coil actuators and the like, maybe used to position grating 14 in accordance with the invention.

[0029] Grating 14 is positioned in optical path 28 and provides opticalfeed back to gain medium 12 along path 28. Reflective grating face 20includes a plurality of ridges, grooves or other diffractive features(not referenced) thereon that are dimensioned and configured to diffractlight beam 24. In the embodiment shown in FIG. 1, grating 14 and gainmedium 12 are positioned in a “Littrow” configuration, although otherconfigurations or arrangements of grating 14 and gain medium 12 mayalternatively be used with the invention. Grating 14 is movable, and maybe positionally adjusted to select the wavelength of light that is fedback to gain medium 12 by grating 14, the phase of the feedback light togain medium 12, and the amount or optical power of the feedback to gainmedium 12 by positional adjustment grating 14. The positional adjustmentof grating 14 in accordance with the invention allows wavelengthselection and phase adjustment of the feedback to gain medium 12, byselective positional adjustment of grating, to be carried outorthogonally or independently, such that wavelength selection adjustmentdoes not affect or minimally effects phase adjustment, and vice versa.

[0030] Grating 14 is movable with respect to a first degree of freedomof motion to provide wavelength selection. In the embodiment shown,grating 14 is rotatable about a first axis A (normal to page) that isparallel to or substantially parallel to grating face 20, in order toprovide for wavelength selection. The diffractive nature of grating 14imparts a spatial separation to light of different wavelengths in beam24, and rotatable adjustment of grating 14 about axis A controls theparticular diffraction, and hence the particular wavelength, that isreturned or fed back to gain medium 12.

[0031] Positional adjustment of grating 14 with respect to a seconddegree of freedom of movement can provide for adjustment of the phase ofthe light fed back to gain medium 12 by grating 14. In the embodiment ofFIG. 1, grating 14 is translatably adjustable with respect to a secondaxis B. Translation of grating 14 along axis B or in the direction ofaxis B alters the distance between grating face 20 and gain medium facet18, and thus alters the length of external cavity 22. Axis B as shown isperpendicular or substantially perpendicular or normal to axis A and tograting face 14.

[0032] External cavity 22 defines a plurality of cavity modes (notshown) that result in transmission maxima that are periodically spacedapart in wavelength. During laser operation at a selected wavelength,lasing may jump or “hop” to an external cavity mode that is adjacent tothe selected wavelength. Such “mode hops” are generally undesirable, andadjustment of the length of external cavity 22 are made during laseroperation to optimally position the external cavity modes with respectto the selected wavelength and thus avoid unwanted mode hopping.Selective translational actuation of grating 14 along axis B adjusts thelength of external cavity 22 and provides a phase adjustment to thelight fed back to gain medium 12 from grating 14, and serves to positionthe external cavity modes so that mode hopping is avoided. This phaseadjustment can also be used to provide fine tuning of the selectedwavelength.

[0033] Translation of grating 14 along axis B to provide phase controlfor positioning of external cavity modes can be carried out withoutvarying the rotation of grating 14 with respect to axis A. Phaseadjustment and wavelength adjustment thus can be easily orthogonalizedor made independent of each other in the apparatus 10, and wavelengthadjustment for the laser apparatus 10 by angle tuning of grating 14 canbe made without effecting, or minimally effecting, external cavitylength. Similarly, translation of grating 14 along axis B to adjustexternal cavity length does not affect the orientation of grating withrespect to axis A. The orthogonal tuning of a wavelength selectionelement and an external cavity length tuning element in an externalcavity laser is also described in U.S. patent application Ser. No.09/900,373 filed on Jul. 6, 2001, the disclosure of which isincorporated herein by reference.

[0034] Grating 14 may also be positionally adjustable with respect to athird degree of freedom of motion to control the amount or level offeedback to gain medium 12 from grating 14, and hence control the outputpower level of the apparatus 10. In the apparatus 10, rotatable motionof grating 14 about axis C provides this third degree of freedom ofmotion. Rotation of grating 14 about axis C alters the alignment of thereturn beam (not shown) towards gain medium and thus can control theamount of level of light fed back to gain medium 12. Rotation of grating14 about axis C also changes the orientation of grating 14 with respectto the polarization of light beam 24, which also effects the level offeedback. Rotation of grating 14 about axis C does not effect or alterthe orientation or angle of grating with respect to axis A, and thusdoes not effect wavelength selection. Rotation of grating 14 withrespect to axis C during laser operation provides several advantages,which are discussed further below.

[0035] The apparatus 10 includes a filter element that provides forsuppression of lasing by the apparatus 10 at unwanted wavelengths. Inthe embodiment of FIG. 1, the filter element is shown as a Fabry-Perotetalon 41 positioned in optical path 28. Etalon 41 includes partiallyreflective faces 42, 44 that, together with the refractive index ofetalon 41, are configured to define a plurality of transmission peaks(not shown) for light beam 24. In this regard, etalon 41 serves as awavelength locker or grid generator, with the plurality of transmissionpeaks of etalon 41 corresponding to discrete selectable wavelengths thatmay be chosen or selected by rotation of grating about axis A as relatedabove. The transmission peaks of etalon 41 thus, for example, maycorrespond in wavelength to the wavelengths of the InternationalTelecommunications Union (ITU) grid channels. Lasing at wavelengthsother than wavelengths corresponding to the transmission peaks of etalon41 are suppressed. Etalon 41 thus provides for suppression of externalcavity modes at unwanted wavelengths.

[0036] In some embodiments of the invention, the length of externalcavity 22 may be sufficiently short such that effective external cavitymode suppression is achieved without the presence of etalon 41. Incertain embodiments, etalon 41 may be actively tuned during laseroperation to vary the free spectral range of etalon 41, and hence thewavelength location of transmission peaks defined by the etalon 41.Tuning of etalon may be carried out mechanically via tilt adjustment orvia thermo-optic, electro-optic, acousto-optic or other mechanism wherethe material of etalon 41 has a refractive index that is responsive totemperature, voltage or other controllable property. The active tuningof a grid generator etalon is described more fully in U.S. patentapplication Ser. No. 09/900,474 filed on Jul. 6, 2001, the disclosure ofwhich is incorporated herein by reference.

[0037] Referring to FIG. 2 and FIG. 3 as well as FIG. 1, the laserapparatus 10 is shown embodied in the apparatus 46, wherein like partsare denoted with like reference numbers. In the apparatus 46, theexternal cavity laser 10 is enclosed in a hermetically sealablecontainer 48. The lid (not shown) of container 48 is omitted forclarity. Container 48 allows the laser 10 to be sealed within an inertatmosphere to prevent contamination and/or degradation of opticalsurfaces on the various components of laser 10, and particularly theanti-reflection coating on facet 16 (not shown in FIG. 2 and FIG. 3) ofgain medium. A tubular support 50 holds an optical fiber (not shown) andallows the optical fiber to communicate with the hermetically sealedinterior of container 48. A ferrule 52 is provided to position the endof the optical fiber so that output from the laser apparatus 10 may befocused by lens 36 into the fiber. The use of an external cavity laserin a hermetically sealable enclosure is also described in U.S. patentapplication Ser. No. 09/900,423 filed on Jul. 6, 2001, the disclosure ofwhich is incorporated herein by reference.

[0038] As shown in FIG. 2 and FIG. 3, gain medium 12 and opticalisolator 40 are configured for selective thermal control independentlyfrom etalon 41 and grating 14. Gain medium 12 and optical isolator 40are mounted on a thermally conductive platform or pad 54. Lenses 26, 36and ferrule 52 may also be mounted on platform 54 as shown. Platform 54in turn is mounted on a thermal control element 56, which may comprise aconventional thermoelectric controller or TEC. Thermal control element56 allows selective thermal control of gain medium 12 and opticalisolator during laser operation. The use of selective thermal control ofa gain medium and optical isolator in an external cavity laser is alsodescribed in U.S. patent application Ser. No. 09/900,429 filed on Jul.6, 2001, the disclosure of which is incorporated herein by reference.Etalon 41 is mounted on a thermally conductive platform 58, which inturn is mounted on a separate thermal control element 60, to allowactive thermal control of etalon 41 independently from the thermalcontrol of gain medium 12 and isolator 40.

[0039] A plurality of electrical leads communicate 62 with the interiorof container 48 to provide power to gain medium 12, to MEMS actuator 19,to thermoelectric controllers 56, 61, and to logic elements and/orcircuitry (not shown) that is associated with controlling the currentdelivered to gain medium, control of thermoelectric controllers 56, 60,and control of the degrees of freedom of motion with respect to axes Aand B provided by MEMS actuator 19. Container 48 includes flanges 64 toallow mounting of the apparatus 46 onto suitable surfaces (not shown).

[0040] The MEMS actuator 19 provides for rotational motion of grating 14about axis A, translation motion with respect to axis B, and rotationalmotion about axis C as noted above. MEMS rotational and translationalactuation may be carried out through a variety of mechanisms, includingmechanical, electrostatic, piezoresistive, piezoelectric, thermoelectricelectromagnetic and/or other interaction of micromachined parts.Micromachining may involve, for example, conventional photolithographic,material deposition, etching, polishing, plating and other techniquesused in semiconductor device manufacture, to form actuator components.The fabrication and use of various types of MEMS devices are known inthe art and are described, for example, in “Introduction toMicroelectromechanical Systems Engineering” by Nadim Maluf and publishedby Artech House, Inc., Norwood Mass. (2000), the disclosure of which isincorporated herein by reference.

[0041] Numerous micromachined MEMS rotational and translational driverconfigurations may be used with the invention. Referring moreparticularly to FIG. 4, as well as FIG. 1 through FIG. 3, MEMS actuator19 is shown as includes one or more drive elements 66 which arecontained or housed in a carrier 68. Drive element 66 is coupled tocarrier 68 by one or more spring elements, hinge elements, gimbalelements, other movable elements (not shown) that are internal tocarrier 68 and which movably connect drive element 66 to carrier 68.Drive element 66, carrier 68, and the movable connecting elementstherebetween may be fabricated from the same substrate or from differentcomponents. Drive element 66 is movable with respect to carrier viaactuation according to mechanical, electrostatic, piezoresistive,piezoelectric, thermoelectric electromagnetic and/or other effect asnoted above.

[0042] Drive element 66 and carrier 68 are configured to provide a rangeof movement to grating 14 that includes rotation about axis A,translation along axis B, and rotation about axis C as noted above. Inthis regard, drive element 66 is operatively coupled to a transmittersubstrate 70, and transmitter substrate 70 is operatively coupled to aMEMS engine element 72. Grating 14 is provided by the reflective surface20 of MEMS engine 72, which may comprise a polished surface ofsemiconductor substrate material. A plurality of grating lines, groovesor other diffractive features (not referenced) are included on surface20 to define grating 14, and may be formed on surface 20 viaconventional photolithographic techniques or other methods. The gratinglines as shown in FIG. 4 and the other FIGs. herein are onlyillustrative and are not shown to scale, and are not necessarilyindicative of grating orientations that would be used during laseroperation.

[0043] MEMS actuator 19 as described above provides positionaladjustment to grating along three degrees of freedom of movement using asingle actuator device. In certain embodiments, multiple MEMS actuatorsmay be utilized, with each actuator configured to provide a desiredpositional degree of freedom to grating 14. Thus, three separate MEMSactuator devices may be used in place of the single device 19, with oneMEMS actuator configured to provide rotational adjustment of grating 14about axis A, another MEMS actuator configured to provide translationaladjustment of grating 14 along axis B, and a third MEMS actuator used toprovide rotational adjustment to grating 14 about axis C. Variousarrangements and configurations of MEMS actuators for positioning ofgrating 14 in accordance with the invention are possible, and willsuggest themselves to those skilled in the art.

[0044] Referring again to FIG. 1 in particular, in the operation oflaser apparatus 10, gain medium 12 is current-pumped in a conventionalmanner, and emits beam 24 from anti-reflection coated facet 16 whichcollimated along path 28 and directed to grating 14. Grating 14 returnsor feeds back light of a selected wavelength (according to the angle ofgrating with respect to axis A) along path 28 to gain medium 12 toprovide lasing at the selected wavelength. Etalon 41 creates a pluralityof transmission peaks, and the selected wavelength corresponds with oneof these transmission peaks. Lasing at other wavelengths, which mayarise from the presence of external cavity modes as described above, issuppressed by etalon 41. During lasing, a portion of the optical outputfrom gain medium 12 exits partially reflective facet 18. This output iscollimated and directed along output path 34 through optical isolator40, and then focused into fiber 38 for use.

[0045] When a change in the lasing wavelength is desired, MEMS actuator19 drives grating 14 rotatably with respect to axis A to change thediffraction, and hence the selected wavelength, that is returned to gainmedium. A controller element (not shown) may be used in association withMEMS actuator 19 to provide control signals thereto to drive or actuategrating 14 to positions corresponding to desired or selectedwavelengths. The relation of grating position with selectablewavelengths may be embodied in a stored lookup table that is consultedby the controller when a change in wavelength selection is made. Therelatively small size of MEMS actuator 19 allows rapid wavelengthtuning. Using the apparatus of FIG. 1, wavelength tuning across a 40nanometer channel spacing at sub-microsecond times, and channelswitching on the order of milliseconds, are achievable.

[0046] MEMS actuator 19 may additionally translate grating 14 along axisB to “trim” or otherwise adjust the length of external cavity 28 toprovide fine tuning of the selected wavelength and to prevent modehopping to external cavity modes adjacent to the selected wavelength asdescribed above. Translational adjustment of grating 14 along axis B maybe made via use of a servo system (not shown). In such a servo system,for example, a sensor is used to monitor the output power of theapparatus 10, either by optically monitoring output or by electricallymonitoring of voltage across gain medium 12 during laser operation.Detection of non-optimal output power gives rise to error signals, whichare then used by the servo system to drive MEMS actuator 19 to translategrating 14 along axis B until optimal output power is detected. The useof servo systems to control external cavity length are disclosed in U.S.patent application Ser. Nos. 09/900,426 and 09/09/900,443, both filed onJul. 6, 2001, the disclosures of which are incorporated herein byreference.

[0047] During the operation of the laser apparatus 10, grating 14 willtypically be oriented to maximize the level of feed back to gain medium12 at the selected wavelength. In many instances, however, it may bedesirable to vary the orientation of grating 14 with respect to thepolarization orientation of light beam 24 during laser operation tocontrol the amount of level of feedback to gain medium 12 from grating14, and hence control the output power level of the apparatus 10. Thisis achieved by rotation of grating 14 about axis C by MEMS actuator 19.Active control of the pitch of grating 14 by rotation of grating aboutaxis C provides rapid, accurate control of the output power level of theapparatus.

[0048] Rotation of grating 14 with respect to axis B allows the outputfrom apparatus 10 to be “turned off” without actually powering down thediode gain medium 12. The output of laser apparatus 10 can thus bebriefly interrupted, by pitch adjustment of grating with respect to axisC, without power down of gain medium, while wavelength selectionadjustment is made by rotational adjustment of grating 14 with respectto axis A. Due to the small size of MEMS actuator 19 and rapidrotational movement that it can impart to grating 14, temporary “powerdowns” for the apparatus on the order microsecond duration or less canbe achieved.

[0049] Active pitch control of grating 14 by rotation of grating 14 withrespect to axis B allows the apparatus 10 to provide a steady level ofoutput power in situations where environmental fluctuation may causesunwanted variation in laser output power. One such source ofenvironmental fluctuation is current fluctuation when the current supplyto gain medium 12 is uneven or not “clean”. During external cavity laseroperation, spurious fluctuation in the level of current delivered to thelaser gain medium causes unwanted fluctuation in the level of outputpower from the laser. Current fluctuation can be controlled using a“roll off” filter to provide a “clean” current to the gain medium. Theuse of such a filter, however, prevents active control of the currentdelivered to the laser gain medium. The active pitch control of grating14 by MEMS actuator 19 allows steady laser output power without the useof a filter to provide a clean current.

[0050] Pitch control of grating 14 as described above may also be usedto maintain a steady or even level of power output over the lifetime ofa laser apparatus. The pitch of grating 14 with respect to axis C may beinitially adjusted to provide a sub-maximum output power. As the diodegain medium 12 and antireflection coating 14 on facet 16 age anddeteriorate due to repeated use, the level of output power achievablefrom the apparatus 10 will drop off. Period adjustment of the pitch ofgrating 14 to maintain a constant output power level avoids this effect.Pitch control of grating 14 with respect to axis C also may be usedduring manufacture or assembly of the apparatus.

[0051] Active pitch control of grating 14 to provide a constant outputpower may involve use of a servo mechanism (not shown) to monitor laseroutput power and make corresponding adjustments to the pitch of grating14. The servo mechanism may involve monitoring of output power of theapparatus 10, with detection of non-optimal output power resulting inerror signals that are then used by the servo system to drive MEMSactuator 19 to rotate grating 14 about axis C until optimal output poweris again detected. Monitoring of output power may be carried optically,or electrically by monitoring voltage across gain medium 12, as relatedabove. Servoing of grating pitch to output power may also be carried outby introducing a frequency dither or modulation to the pitch of grating,and then monitoring the frequency dither. The use of servo systems usingintroduction of a frequency dither to an optical element to controlexternal cavity length are disclosed in U.S. patent application Ser.Nos. 09/900,426 and 09/09/900,443, filed on Jul. 6, 2001 andincorporated herein by reference.

[0052] Reference is now made to FIG. 5, wherein another embodiment of anexternal cavity laser apparatus 74 is shown, with like reference numbersused to denote like parts. In the apparatus 74, etalon 41 external tocavity 22, and is positioned in output beam 40. Etalon 41 may be locatedbefore isolator 40, or elsewhere in output path 34. Grating 14 isoperative coupled to a MEMS actuator 19 as described above, and ispositionable by MEMS actuator 19 to control wavelength selection,external cavity length and power level as described above. Etalon 41operates in the same was when in an intracavity location as shown inFIG. 1, by defining selectable transmission bands or peaks, andproviding for suppression of lasing at wavelengths other than theselectable transmission bands. In other respects, the operation of laserapparatus 74 is substantially the same as described above for theapparatus 10 of FIG. 1.

[0053] Referring now to FIG. 6, there is shown still another embodimentof an external cavity laser apparatus 76 in accordance with theinvention, with like reference numbers used to denote like parts. In theapparatus 76, a movable reflective element or mirror 78 is operativelycoupled to MEMS actuator 19. Movable mirror 78 reflects beam 24 to astationary grating element 80, such that optical path 28 extends frommirror 78 to grating 80. Grating 80 includes a reflective surface 82with a plurality of grating lines (not referenced) etched thereon toprovide for diffraction of light beam 24 in the manner described above.The external cavity (not referenced) in the apparatus 76 is defined ordelineated by grating surface 80 and facet 18 of gain medium 12, suchthat the external cavity is “folded” about mirror 78.

[0054] Mirror 78 is movable with respect to optical path 28 according tooperation of MEMS actuator 19, and is rotatable about axis A,translatable along axis B, and rotatable about axis B in the same manneras the grating 14 in the apparatus 10 described above. Rotation ofmirror about axis A by MEMS actuator 19 alters the diffraction fromstationary grating 80 that is returned from grating 80 to mirror 78, andthus provides for selection of a wavelength of light that is fed back togain medium 12 by mirror 78 along optical path. Translation of mirroralong axis B by MEMS actuator 19 adjusts the length of the externalcavity and allows cavity “trimming” adjustment to avoid unwanted modehopping and to fine tune selected feedback wavelength as describedabove. Rotation of mirror 78 about axis C by MEMS actuator 19 alters theamount of light that is returned from grating 80 to mirror 78, and hencecontrols the amount or level of feed back that is returned from mirror78 to gain medium 12.

[0055] The apparatus 76 thus operates in a manner that is similar to theapparatus 10 and 74 described above, with the primary exception beingthat positional adjustment of an intracavity mirror 78 with respect to astationary grating 80, rather than positional adjustment of a grating,is used to control the wavelength selection, external cavity length, andpower attenuation operations described above. The same degrees offreedom of movement of mirror 78 are used to perform the wavelengthselection control, external cavity length control, and power attenuationcontrol, as described above for the grating 14 of apparatus 10. In otherrespects the operation of the laser apparatus 76 is substantially thesame as described above for the apparatus 10.

[0056] The laser apparatus 78 may also be embodied in a lasertransmitter device 84 as shown in FIG. 7 wherein the apparatus 78 isenclosed within a hermetically sealable container or enclosure 48. Likeparts in FIG. 7 are denoted by like reference numbers. As in theapparatus 46 described above, gain medium 12 and isolator 40 are mountedon thermally conductive platform 54 for selective thermal control bythermoelectric controller 56, and etalon 41 is mounted on thermallyconductive platform 58 for independent thermal control by thermoelectriccontroller 60. Output from laser apparatus 78 is directed into a fiber(not shown) mounted in ferrule 52 as also described above. The overallpackage size of the laser device 84 and container 48 of FIG. 7 may beslightly larger than that of the apparatus 46 in FIG. 2, in order toaccommodate the longer external cavity of the laser apparatus 78.

[0057] Referring now to FIG. 8, there is shown a laser apparatus 84,with like numbers denoting like part. In the apparatus 84, thestationary reflective grating 80 is changed in orientation with respectto movable mirror 78. In the apparatus 76 and 78 discussed above,grating 80 is shown in an orientation such that grating surface 82 issubstantially parallel to movable mirror 78. In the apparatus 84 of FIG.8, grating 80 is positioned such that grating face 82 is substantiallyperpendicular or normal to mirror 78. The apparatus 84 in other respectsis substantially identical to the apparatus 84 described above.

[0058] Referring next to FIG. 9, yet another embodiment of a laserapparatus 86, wherein like numbers are used to denote like parts. In theapparatus 86, the etalon filter 41 is omitted, and grating 14 ispositioned proximate to collimator 26, to shorten the length of theexternal laser cavity defined by grating surface 20 and rear facet 18 ofgain medium. Shortening of the laser cavity in this manner increases thespacing of the external cavity modes with respect to each other. Thus,the external cavity modes may be configured such that no external cavitymodes are proximate to or otherwise close in wavelength to theselectable wavelengths of the laser apparatus 86.

[0059] The short external laser cavity length of the apparatus 86eliminates the need for an additional wavelength suppression element orfilter, such as the etalon 41 shown in FIG. 1 through FIG. 8 anddiscussed above, because unwanted lasing associated with external cavitymodes will not occur due to the relatively wide spacing of the cavitymodes. In some embodiments, the external laser cavity length can beadjusted such that the external cavity modes themselves define thewavelengths that are selected by positioning of grating 14. In otherwords, the external cavity serves as a wavelength locker or gridgenerator, with the cavity mode peaks corresponding to a desiredwavelength grid. Omission of the etalon filter also allows a smalleroverall package for the apparatus 86, and reduces the overall cost ofthe apparatus.

[0060] In other embodiments of the invention, a tunable wavelengthselection other than a grating may be used. Thus, MEMS actuator 19 maybe configured to position an etalon, interference filter, prism or otherelement that can provide wavelength selection according to MEMS actuatedpositioning. Various types of tunable wavelength selection elements thatmay be used with the invention in place of grating 14 are disclosed inU.S. patent application Ser. No. 09/814,464 filed on Mar. 21, 2001, thedisclosure of which is incorporated herein by reference.

[0061] While the present invention has been described with reference tothe specific embodiments thereof, it should be understood by thoseskilled in the art that various changes may be made and equivalents maybe substituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, material, composition of matter, process, processstep or steps, to the objective, spirit and scope of the presentinvention. All such modifications are intended to be within the scope ofthe claims appended hereto.

What is claimed is:
 1. An external cavity laser apparatus, comprising:(a) a gain medium emitting a light beam; and (b) a tunable elementpositioned in said light beam and configured to feed back light of aselected wavelength to said gain medium; (c) said tunable elementadjustable according to a first degree of freedom of movement to provideselection of wavelength of said light fed back to said gain medium; (d)said tunable element adjustable according to a second degree of freedomof movement to provide control of phase of said light fed back to saidgain medium; (e) said tunable element configured such that adjustment ofsaid tunable element for said selection of said wavelength is orthogonalwith respect to adjustment of said tunable element for control of saidphase.
 2. The apparatus of claim 1, further comprising amicroelectromechanical system actuator element operatively coupled tosaid tunable element, said microelectromechanical system actuatorelement configured to actuate said tunable element according to saidfirst and second degrees of freedom of movement.
 3. The apparatus ofclaim 2, wherein said microelectromechanical system actuator isconfigured to actuate said tunable wavelength selection elementaccording to a third degree of freedom of movement to provide adjustmentof power of said light fed back to said gain medium.
 4. The apparatus ofclaim 1, wherein said tunable wavelength selection element comprises amovable grating, said grating and a facet of said gain medium definingan external laser cavity.
 5. The apparatus of claim 4, wherein saidmicroelectromechanical system actuator element is configured to rotatesaid grating about a first axis to control wavelength of said light fedback to said gain medium by said grating.
 6. The apparatus of claim 5,wherein said microelectromechanical system actuator element isconfigured to translate said grating along a second axis to controllength of said external cavity.
 7. The apparatus of claim 5, whereinsaid microelectromechanical system actuator element is configured torotate said grating about a third axis to control attenuation of saidlight fed back to said gain medium by said grating.
 8. The apparatus ofclaim 7, wherein said first axis is parallel to a reflective face ofsaid grating.
 9. The apparatus of claim 8, wherein said second axis isperpendicular to said reflective face of said grating.
 10. The apparatusof claim 9, wherein said third axis is perpendicular to said first axisand said second axis.
 11. The apparatus of claim 1, wherein said tunablewavelength selection element comprises a movable mirror and a stationaryreflective grating.
 12. The apparatus of claim 11, wherein saidmicroelectromechanical system actuator element is configured to rotatesaid mirror about a first axis to control wavelength of said light fedback to said gain medium.
 13. The apparatus of claim 12, wherein saidmicroelectromechanical system actuator element is configured totranslate said mirror along a second axis to control external cavitylength.
 14. The apparatus of claim 12, wherein saidmicroelectromechanical system actuator element is configured to rotatesaid mirror about a third axis to control attenuation of said light fedback to said gain medium.
 15. The apparatus of claim 1, furthercomprising a filter element positioned in said light beam, said filterelement configured to suppress lasing at wavelengths other than saidselected wavelength.
 16. The apparatus of claim 1, wherein a reflectiveface of said tunable wavelength selection element and a facet of saidgrating define an external laser cavity.
 17. The apparatus of claim 16,wherein said external laser cavity is dimensioned to define a pluralityof external cavity modes, said external cavity modes corresponding toselectable wavelengths.
 18. An external cavity laser apparatus,comprising: (a) a gain medium having first and second facets andemitting a light beam from said first facet; (b) a tunable wavelengthselection element, said tunable wavelength selection element comprisinga grating, said tunable wavelength selection element positioned in saidlight beam and configured feed back light to said gain medium, saidgrating and said second facet of said gain medium defining an externallaser cavity; and (c) a microelectromechanical system actuator elementoperatively coupled to said tunable wavelength selection element; (d)said microelectromechanical system actuator element configured toactuate said tunable wavelength selection element according to a firstdegree of freedom of movement to provide selection of wavelength of saidlight fed back to said gain medium; (e) said microelectromechanicalsystem actuator configured to actuate said tunable wavelength selectionelement according to a second degree of freedom of movement to controllength of said external laser cavity; (f) said tunable wavelengthselection element configured such that said adjustment of said selectionof said wavelength is orthogonal with respect to said adjustment of saidphase.
 19. The apparatus of claim 18, wherein saidmicroelectromechanical system actuator is configured to actuate saidtunable wavelength selection element according to a third degree offreedom of movement to control power of said light fed back to said gainmedium.
 20. The apparatus of claim 19, wherein: (a) saidmicroelectromechanical system actuator is operatively coupled to saidgrating; (b) said microelectromechanical system actuator elementconfigured to rotate said grating about a first axis to control saidwavelength of said light fed back to said gain medium; (c) saidmicroelectromechanical system actuator element configured to translatesaid grating along a second axis to control said length of said externallaser cavity; and (d) said microelectromechanical system actuatorelement configured to rotate said grating about a third axis to controlsaid power of said light fed back to said gain medium.
 21. The apparatusof claim 20, wherein said first axis is parallel to a reflective face ofsaid grating.
 22. The apparatus of claim 21, wherein said second axis isperpendicular to said reflective face of said grating.
 23. The apparatusof claim 19, wherein: (a) said tunable wavelength selection elementfurther comprises a mirror, said mirror operatively coupled to saidmicroelectromechanical system actuator; (b) said microelectromechanicalsystem actuator element configured to rotate said mirror about a firstaxis to control said wavelength of said light fed back to said gainmedium; (c) said microelectromechanical system actuator elementconfigured to translate said mirror along a second axis to control saidlength of said external laser cavity; and (d) saidmicroelectromechanical system actuator element configured to rotate saidmirror about a third axis to control said power of said light fed backto said gain medium.
 24. The apparatus of claim 23, wherein said firstaxis is parallel to a reflective face of said grating.
 25. The apparatusof claim 24, wherein said second axis is perpendicular to saidreflective face of said grating.
 26. The apparatus of claim 18, furthercomprising a filter element positioned in said light beam, said filterelement configured to suppress lasing at wavelengths other than saidselected wavelength.
 27. The apparatus of claim 18, wherein saidexternal laser cavity is dimensioned to define a plurality of externalcavity modes, said external cavity modes corresponding to selectablewavelengths.
 28. The apparatus of claim 18, wherein said external lasercavity is dimensioned to suppress lasing associated with external cavitymodes.
 29. A method for laser operation, comprising: (a) emitting alight beam from a gain medium; (b) positioning a tunable element in saidlight beam; (d) feeding back light to said gain medium by said tunableelement; (e) selecting wavelength of said light fed back to said gainmedium by positionally adjusting said tunable element with respect to afirst degree of freedom of movement; and (f) controlling phase of saidlight fed back to said gain medium by positionally adjusting saidtunable element with respect to a second degree of freedom of motion;(g) said positionally adjusting of said tunable element for selectingsaid wavelength carried out orthogonally with respect to saidpositionally adjusting said tunable element for controlling said phaseof said light fed back to said gain medium.
 30. The method of claim 29,further comprising coupling said tunable wavelength selection element toa microelectromechanical actuator.
 31. The method of claim 30, furthercomprising adjusting power of said light fed back to said gain medium bypositionally adjusting said tunable wavelength selection element, withsaid microelectromechanical actuator, with respect to a third degree offreedom of movement.
 32. The method of claim 31, wherein saidpositioning a tunable wavelength selection element in said light beamcomprises positioning a movable grating in said light beam, said gratingand a reflective facet of said gain medium defining an external lasercavity.
 33. The method of claim 32, wherein said selecting saidwavelength of said light fed back to said gain medium comprisesrotatably adjusting said grating, by said microelectromechanicalactuator, about a first axis.
 34. The method of claim 33, wherein saidadjusting said phase of said light fed back to said gain mediumcomprises adjusting length of said external cavity by translatablyadjusting said grating, by said microelectromechanical actuator, along asecond axis.
 35. The method of claim 34, wherein said adjusting saidpower of said light fed back to said gain medium comprises rotatablyadjusting said grating, by said microelectromechanical actuator, about athird axis.
 36. The method of claim 31, wherein said positioning atunable wavelength selection element in said light beam comprisespositioning a movable mirror in said light beam, and positioning astationary grating in said light beam after said movable mirror, saidgrating and a reflective facet of said gain medium defining an externallaser cavity.
 37. The method of claim 36, wherein said selecting saidwavelength of said light fed back to said gain medium comprisesrotatably adjusting said mirror, by said microelectromechanicalactuator, about a first axis.
 38. The method of claim 37, wherein saidadjusting said phase of said light fed back to said gain mediumcomprises adjusting length of said external cavity by translatablyadjusting said mirror, by said microelectromechanical actuator, along asecond axis.
 39. The method of claim 38, wherein said adjusting saidpower of said light fed back to said gain medium comprises rotatablyadjusting said mirror, by said microelectromechanical actuator, about athird axis.
 40. The method of claim 29, further comprising positioning afilter element in said light beam, said filter element configured tosuppress lasing at wavelengths other than a selected wavelength.
 41. Anlaser apparatus, comprising: (a) gain means for emitting a light beam(b) tunable means for wavelength selection positioned in said light beamand configured feed back light of a selected wavelength to said gainmedium; (c) microelectromechanical system actuation means forpositionally adjusting said tunable means with respect to a first degreeof freedom of movement to provide selection of wavelength of said lightfed back to said gain medium, and for positionally adjusting saidtunable means with respect to a second degree of freedom of movement toprovide control of phase of said light fed back to said gain medium; (d)said tunable means configured such that said adjustment of said tunablemeans for selection of said wavelength is orthogonal with respect tosaid adjustment of said tunable means for control of said phase.